CO CO Materials Science & Fabrication Techniques 2 — Questions and Answers
Question 1: Which casting technique for a foot orthosis involves maintaining the subtalar joint in neutral position while allowing the forefoot to align naturally?
- Weight-bearing foam box impression
- Non-weight-bearing subtalar neutral cast (Correct answer)
- Semi-weight-bearing plaster slipper cast
- Dynamic pedobarograph scan
Correct answer: Non-weight-bearing subtalar neutral cast
The non-weight-bearing subtalar neutral cast captures the foot with the subtalar joint held in neutral, providing a biomechanically accurate model for custom orthosis fabrication.
Question 2: What is the purpose of adding a metatarsal pad to a foot orthosis?
- To increase heel cushioning and reduce plantar fascia tension
- To offload the metatarsal heads by redistributing pressure proximally (Correct answer)
- To correct rearfoot varus alignment
- To limit first MTP joint dorsiflexion
Correct answer: To offload the metatarsal heads by redistributing pressure proximally
A metatarsal pad placed proximal to the metatarsal heads redistributes pressure away from them, alleviating pain from metatarsalgia or Morton's neuroma.
Question 3: Closed-cell foam such as Plastazote is commonly used as an orthotic interface material primarily because it:
- Provides rigid structural support for the arch
- Is heat-moldable and provides cushioning with pressure distribution (Correct answer)
- Has high tensile strength resistant to tearing
- Allows moisture to wick away from the skin
Correct answer: Is heat-moldable and provides cushioning with pressure distribution
Plastazote is a closed-cell polyethylene foam that can be heat-molded to conform to the foot and provides excellent cushioning and pressure distribution for insensate or painful feet.
Question 4: A rectified positive model is used in orthotic fabrication to:
- Create an exact replica of the patient's foot for diagnostic purposes only
- Modify the plaster cast to build in corrections or accommodations before forming the device (Correct answer)
- Test the fit of the orthosis before delivering it to the patient
- Record the patient's weight-bearing footprint
Correct answer: Modify the plaster cast to build in corrections or accommodations before forming the device
A rectified positive model is a plaster model that has been modified by adding or removing material to build in the desired biomechanical corrections before the orthosis is formed over it.
Question 5: Which rivet or fastener type is most appropriate for attaching metal uprights to a leather orthotic foundation to allow articulation?
- Pop rivet (blind rivet)
- Chicago screw (sex bolt) (Correct answer)
- Solid brass rivet
- Stainless steel self-tapping screw
Correct answer: Chicago screw (sex bolt)
Chicago screws (sex bolts) are ideal for orthotic joints because they can be tightened to control resistance to motion and can be replaced if worn, unlike solid rivets.
Question 6: What is the primary reason stainless steel alloys are preferred over aluminum alloys for load-bearing orthotic uprights in heavier patients?
- Stainless steel is lighter and reduces the overall weight of the device
- Stainless steel has greater yield strength and fatigue resistance under high loads (Correct answer)
- Stainless steel is easier to bend and shape during fabrication
- Stainless steel is less expensive than aluminum alloys
Correct answer: Stainless steel has greater yield strength and fatigue resistance under high loads
Stainless steel has higher yield strength and superior fatigue resistance compared to aluminum, making it the appropriate choice for uprights that must bear significant repetitive loads without deforming.
CO CO Materials Science & Fabrication Techniques 2
Which casting technique for a foot orthosis involves maintaining the subtalar joint in neutral position while allowing the forefoot to align naturally?